Aqueous polymer dispersion having a broad particle size distribution

ABSTRACT

An aqueous polymer dispersion with a final solids volume concentration of at least 50% by volume is obtainable by polymerizing at least one radical polymerizable monomer by the method of free radical aqueous emulsion polymerization with the addition of an aqueous dispersion of a starting polymer.

This is a division, of application Ser. No. 08/038,864,filed on Mar. 29,1993, now U.S. Pat. No. 5,350,787.

The present invention relates to final aqueous polymer dispersions whichhave a solids volume concentration (solids volume=solids mass divided bysolids density) of at least 50% by volume and are obtainable bypolymerizing at least one radical polymerizable monomer with theaddition of at least one aqueous dispersion of a starting polymer in apolymerization vessel by the method of free radical aqueous emulsionpolymerization in the presence of surface active substances and freeradical polymerization initiators with the proviso that

a) the mass of the at least one radical polymerizable monomer relativeto the total mass formed of the mass of the at least one radicalpolymerizable monomer and the mass of starting polymer added in the formof the at least one aqueous dispersion is from 90 to 99.5%,

b) the starting polymer is present in the at least one aqueousdispersion in the form of dispersed starting polymer particles whosenon-zero weight average particle diameter is equal to or less than 50nm,

c) the total number of starting polymer particles added dispersed in theform of at least one aqueous dispersion of a starting polymer is from10¹⁵ to 10²⁰ starting polymer particles per liter based on the volume ofthe final polymer dispersion,

d) of the total amount of the at least one monomer to be polymerizedeither nothing or at most 10% by weight is introduced into thepolymerization vessel as initial charge prior to the start of the freeradical aqueous emulsion polymerization,

e) of the total amount of the at least one aqueous dispersion of astarting polymer to be added the amount introduced into thepolymerization vessel as initial charge prior to the start of the freeradical aqueous emulsion polymerization is either nil or at most suchthat the total number of starting polymer particles introduced asinitial charge is not more than 10% of the total starting polymerparticles to be added in the form of the at least one aqueousdispersion,

f) of the total amount of the at least one monomer to be polymerized atleast 90% by weight is added to the polymerization vessel after thestart of the free radical aqueous emulsion polymerization, in such a waythat at any time of this addition the polymerization conversion of themonomers already added previously to the polymerization vessel includingthe monomers introduced into the polymerization vessel as initial chargeis at least 80%, based on the total amount formed of the monomers addedpreviously to the polymerization vessel and the monomers introduced intothe polymerization vessel as initial charge,

g) of the total amount of the at least one aqueous dispersion of astarting polymer to be added the proportion added to the polymerizationvessel after the start of the free radical aqueous emulsionpolymerization is at least such that the total number of the startingpolymer particles contained in this added amount of the at least oneaqueous dispersion to be added is at least 90% of the starting polymerparticles to be added in total in the form of the at least one aqueousdispersion of a starting polymer and that this addition is carried outin such a way that

at any time of this addition the polymerization conversion of themonomers already added previously to the polymerization vessel includingthe monomers introduced into the polymerization vessel as initial chargeis at least 80%, based on the total amount formed of the monomers addedpreviously to the polymerization vessel and the monomers introduced intothe polymerization vessel as initial charge;

at any time after the start of the emulsion polymerization the ratioV_(e) of the number of moles of the amount already added previously tothe polymerization vessel of the at least one monomer including theproportion of the at least one monomer introduced into thepolymerization vessel as initial charge to the number of moles of thestarting polymer particles already added previously to thepolymerization vessel in the form of the at least one aqueous dispersionof a starting polymer, standardized for the ratio of the number of molesof the total amount of the at least one monomer to be polymerized to thenumber of moles of the starting polymer particles to be added in totalin the form of the at least one aqueous dispersion of the startingpolymer, is within the range from >0 to 10;

at any time after the start of the emulsion polymerization the ratioV_(a) of the number of moles of the starting polymer particles alreadyadded previously to the polymerization vessel in the form of the atleast one aqueous dispersion of a starting polymer excluding theproportion of the at least one aqueous dispersion of a starting polymerintroduced into the polymerization vessel as initial charge to thenumber of moles of the amount of the at least one monomer already addedpreviously to the polymerization vessel excluding the proportion of theat least one monomer introduced into the polymerization vessel asinitial charge, standardized for the ratio of the number of moles of thestarting polymer particles added in total in the form of the at leastone aqueous dispersion of a starting polymer to the number of moles ofthe amount of the at least one monomer added in total after the start ofthe aqueous emulsion polymerization, is within the range from 0 to 10;

on completion of the addition of the at least one monomer at most 10% ofthe starting polymer particles to be added in total in the form of theat least one aqueous dispersion of a starting polymer are added to thepolymerization vessel;

on completion of the addition of the at least one aqueous dispersion ofa starting polymer at most 30% by weight of the total amount of the atleast one monomer to be polymerized is added to the polymerizationvessel;

h) the final aqueous polymer dispersion contains from 1 to 5% of surfaceactive substances, based on the total mass formed of the mass of the atleast one radical polymerizable monomer and the mass of starting polymerto be added in the form of the at least one aqueous dispersion,

i) of the total amount of surface active substances contained in thefinal polymer dispersion at most 15% by weight is introduced into thepolymerization vessel as initial charge prior to the start of the freeradical aqueous emulsion polymerization (aliquot A),

j) of the total amount of surface active substances contained in thefinal polymer dispersion at least 50% by weight are already part of theat least one aqueous dispersion to be added of a starting polymer(aliquot B),

k) the proportion of the total amount of surface active substancescontained in total in the final polymer dispersion that is neither partof aliquot A nor part of aliquot B (namely aliquot C) is added to thepolymerization vessel after the start of the free radical aqueousemulsion polymerization in such a way that at any time the total amountformed of the proportions of aliquots C and B already present in thepolymerization vessel is less than 5% by weight, based on the totalamount of monomers and polymer present in the polymerization vessel, and

l) the total amount used of free radical polymerization initiators is0.1 to 5% by weight, based on the total amount of the at least onemonomer to be polymerized, and is added to the polymerization vessel inthe course of the free radical aqueous emulsion polymerization in such away that the free radical aqueous emulsion polymerization continues to aminimum polymerization conversion of at least 90% of the total amount ofthe at least one monomer to be polymerized.

The present invention further relates to the process for preparing suchfinal aqueous polymer dispersions and to the use thereof as binders andas materials for preparing coatings and adhesive joints.

Aqueous polymer dispersions are systems comprising polymer particlesdispersed as disperse phase in an aqueous dispersion medium.

Polymer solutions form polymer films as the solvent evaporates. Aqueouspolymer dispersions behave the same way on evaporation of the aqueousdispersion medium, which is why aqueous polymer dispersions find varieduse as binders, for example for paints or for leather coatings.

Aqueous polymer dispersions having a high polymer content are ofparticular advantage in that, on the one hand, their relatively lowerproportion of aqueous dispersion medium reduces the energy required forevaporating it, for example for film formation or for preparing polymerpowders, and, on the other, the useful polymer can be stored andtransported using a relatively smaller amount of aqueous phase ascarrier medium.

However, there is a disadvantage in that, as the volume concentration ofthe polymer increases (U.S. Pat. No. 4,130,523), there are problems withthe preparation of aqueous polymer dispersions. For instance, the flowresistance (the viscosity) increases and this increased viscosity makesit difficult not only to remove the heat of reaction but also to processthe aqueous dispersion; secondly, there is an increasing tendency forthe dispersed polymer particles to aggregate for reasons ofthermodynamic stability. The resulting flocs [a) microflocs or specks;not normally removable by conventional filtration; b) macroflocs orcoagulum; normally removable by conventional filtration] interfere inparticular with the film forming of the aqueous polymer dispersions andare therefore generally undesirable.

According to studies about the flow resistance of aqueous polymerdispersions, those having a broad size distribution (polydispersity) ofthe dispersed polymer particles for the same solids content generallyhave a lower flow resistance than those with a narrow size distribution(which are in the extreme case monodispersed). Furthermore, coarseaqueous polymer dispersions have a lower flow resistance than fineaqueous polymer dispersions, given the same solids content.

EP-A-129 699 discloses a process for preparing an aqueous polymerdispersion wherein unsaturated monomers are polymerized in aconventional manner in a polymerization vessel by the method of freeradical aqueous emulsion polymerization with the addition of an aqueousdispersion of a starting polymer such that the addition of the aqueousdispersion of the starting polymer must be concluded before 40% byweight of the total monomers to be polymerized have copolymerized andmust not start before the average particle size of the emulsion polymerformed in the course of the polymerization of the monomers is twice thatof the aqueous dispersion of the starting polymer. In fact, the aqueousdispersion of the starting polymer is preferably not added over aprolonged period but all at once.

The disadvantages of the aqueous polymer dispersions thus obtainable arethat their flow resistance is not fully satisfactory above a solidsvolume concentration of 50% by volume and that, according to theembodiment examples, the solids volume concentration is limited tovalues below 65% by volume.

U.S. Pat. No. 4,130,523 concerns a process for preparing aqueous polymerdispersions wherein aqueous polymer dispersion already formed in thecourse of the polymerization process is continuously removed from thereaction zone, stored and later reintroduced into the reaction zone as akind of starting polymer dispersion. A disadvantage of this process isthat it is unsuitable for industrial implementation.

U.S. Pat. No. 3,424,706 concerns a process for preparing aqueousdispersions of polymers containing at least 70-97% by weight ofvinylidene chloride as copolymerized units, wherein the polymerizationof the monomers is effected with the addition of an aqueous dispersionof a starting polymer. The said reference teaches inter alia mixing themonomers to be polymerized and the aqueous dispersion of the startingpolymer with one another and adding this mixture to the initial chargecomprising part of the polymerization batch.

The disadvantage with the process of U.S. Pat. No. 3,424,706 is that itis restricted to monomer mixtures consisting chiefly of vinylidenechloride. Moreover, according to the illustrative embodiments, theaqueous polymer dispersions obtainable by this process areunsatisfactory not only as regards the flow resistance above a solidsvolume concentration of 50% by volume but also as regards the upperlimit for the solids volume concentration attainable in a stillsatisfactorily flowable state.

It is an object of the present invention to make available aqueouspolymer dispersions that are obtainable in a simple, industriallysuitable, reproducible manner not restricted to specific monomers withan increased solids volume concentration but a reduced flow resistanceand reduced floc content.

We have found that this object is achieved by the final aqueous polymerdispersions defined at the beginning.

Remarkably, the subject-matter of the invention is not restricted to thefree radical aqueous emulsion polymerization of monomer mixturescomposed chiefly or exclusively of vinyl and/or vinylidene halides,despite the generally known fact that the development of the dispersephase in the case of monomers other than vinyl and/or vinylidene halidesis a significantly more complex phenomenon.

The process of the invention is therefore particularly suitable for,inter alia, monoethylenically unsaturated monomers such as olefins, forexample ethylene, aromatic vinyl monomers such as styrene,α-methylstyrene, o-chlorostyrene or vinyltoluenes, vinyl and vinylidenehalides such as vinyl and vinylidene chloride, esters of vinyl alcoholand monocarboxylic acids having from 1 to 18 carbon atoms, such as vinylacetate, vinyl propionate, vinyl-n-butyrate, vinyl laurate and vinylstearate, esters of α,β-monoethylenically unsaturated mono- anddicarboxylic acids preferably of from 3 to 6 carbon atoms, such as, inparticular, acrylic acid, methacrylic acid, maleic acid, fumaric acidand itaconic acid, with alkanols in general of from 1 to 12, preferablyof from 1 to 8, in particular of from 1 to 4, carbon atoms, such as, inparticular, methyl, ethyl, n-butyl, isobutyl and 2-ethylhexyl acrylateand methacrylate, dimethyl maleate or n-butyl maleate, nitriles ofα,β-monoethylenically unsaturated carboxylic acids such as acrylonitrileand also C₄₋₈ -conjugated dienes such as 1,3-butadiene and isoprene. Themonomers mentioned generally form the principal monomers which, based onthe total amount of the monomers to be polymerized by the method of freeradical aqueous emulsion polymerization, normally account for aproportion of more than 50% by weight. Monomers which polymerized bythemselves normally form homopolymers that possess enhanced watersolubility are normally included in the polymer only as modifyingmonomers, in amounts, based on the total amount of monomers to bepolymerized, of less than 50% by weight, in general from 0.5 to 20,preferably from 1 to 10, % by weight.

Examples of monomers of this type are α,β-monoethylenically unsaturatedmono- and dicarboxylic acids of from 3 to 6 carbon atoms and amidesthereof, e.g. acrylic acid, methacrylic acid, maleic acid, fumaric acid,itaconic acid, acrylamide and methacrylamide, also vinylsulfonic acidand water-soluble salts thereof, and also N-vinylpyrrolidone. Monomerswhich customarily enhance the internal strength of the films formed fromthe final aqueous polymer dispersion are in general likewise included inthe polymer only in minor amounts, usually from 0.5 to 10% by weight,based on the total amount of monomers to be polymerized. Monomers ofthis type normally have an epoxy, hydroxyl, N-methylol, carbonyl or atleast two nonconjugated ethylenically unsaturated double bonds. Examplesthereof are N-alkylolamides of α,β-monoethylenically unsaturatedcarboxylic acids of from 3 to 10 carbon atoms or esters thereof withalcohols of from 1 to 4 carbon atoms, of which N-methylolacrylamide andN-methylolmethacrylamide are particularly preferred, divinyl monomers,divinylidene monomers and also dialkenyl monomers. Particularly suitableinstances of these are the diesters of dihyrdic alcohols withα,β-monoethylenically unsaturated monocarboxylic acids, of which in turnacrylic and methacrylic acid are preferred. Examples of such monomershaving two nonconjugated ethylenically unsaturated double bonds arealkylene glycol diacrylates and dimethacrylates such as ethylene glycoldiacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycoldiacrylate, propylene glycol diacrylate, divinylbenzene, vinylmethacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate,diallyl maleate, diallyl fumarate, methylenebisacrylamide,cyclopentadienyl acrylate and triallyl cyanurate. As well as monomershaving unsaturated double bonds it is possible for minor amounts,customarily from 0.01 to 2% by weight, based on the monomers to bepolymerized, of molecular weight regulators, such as tertdodecylmercaptan to be included in the polymer. It is preferable to add suchsubstances to the polymerization zone mixed with the monomers to bepolymerized.

Suitable surface active substances include not only the protectivecolloids customarily used for carrying out free radical aqueous emulsionpolymerizations but also emulsifiers. Examples of suitable protectivecolloids are polyvinyl alcohols, cellulose derivatives andvinylpyrrolidone-containing copolymers. A detailed description offurther suitable protective colloids may be found in Houben-Weyl,Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe,Georg Thieme Verlag, Stuttgart, 1961, pages 411 to 420. It is of coursealso possible to use mixtures of emulsifiers and/or protective colloids.Preferably, the surface active substances used are exclusivelyemulsifiers whose relative molecular weights are customarily below 1000,in contradistinction to protective colloids. They can be anionic,cationic or nonionic in nature. Of course, if mixtures of surface activesubstances are used, the individual components must be compatible withone another, which can be verified beforehand by means of a fewpreliminary experiments if there is any doubt. In general, anionicemulsifiers are compatible with one another and with nonionicemulsifiers. The same is true of cationic emulsifiers, while anionic andcationic emulsifiers are usually incompatible with one another. Examplesof customary emulsifiers are ethoxylated mono-, di- and trialkylphenols(EO degree: 3-50, alkyl radical: C₄ -C₉), ethoxylated fatty alcohols (EOdegree: 3-50, alkyl radical: C₈ -C₃₆), and also alkali metal andammonium salts of alkyl sulfates (alkyl radical: C₈ -C₁₂), of sulfuricmonoesters of ethoxylated alkanols (EO degree: 4-30, alkyl radical: C₁₂-C₁₈), and ethoxylated alkylphenols (EO degree: 3-50, alkyl radical: C₄-C₉), of alkylsulfonic acids (alkyl radical: C₁₂ -C₁₈) and ofalkylarylsulfonic acids (alkyl radical: C₉ -C₁₈). Further suitableemulsifiers may be found in Houben-Weyl, Methoden der organischenChemie, Volume XIV/1, Makromolekulare Stoffe, Georg Thieme Verlag,Stuttgart, 1961, pages 192 to 208.

Particularly suitable surface active substances are compounds of thegeneral formula I ##STR1## where R¹ and R² are each hydrogen or CC₄ -C₂₄-alkyl but are not both hydrogen, and X and Y are each an alkali metalor ammonium ion. In the formula I, R¹ and R² are each preferably linearor branched alkyl radicals of from 6 to 18 carbon atoms or hydrogen, inparticular of 6, 12 and 16 carbon atoms, but R¹ and R² must not both behydrogen. X and Y are each preferably sodium, potassium or ammoniumions, of which sodium is particularly preferred. Of particular advantageare compounds I in which X and Y are each sodium, R¹ is a branched alkylradical of 12 carbon atoms and R² is hydrogen or R¹. It is common toemploy technical grade mixtures containing from 50 to 90% by weight ofthe monoalkylated product, for example Dowfax® 2A1 (trademark of the DowChemical Company). In the process of the invention compounds I arepreferably used as the sole surface active substances. Compounds I aregenerally known, for example from U.S. Pat. No. 4,269,749, and arecommercially available. It is advantageous for the final aqueous polymerdispersion of the invention to contain from 1 to 3% by weight of surfaceactive substances, based on the mass of the final polymer.

Suitable free radical polymerization initiators are all those which arecapable of initiating a free radical aqueous emulsion polymerization.This includes not only peroxides, for example alkali metalperoxydisulfates, but also azo compounds. Preference is given to usingcombined systems composed of at least one organic reducing agent and atleast one peroxide and/or hydroperoxide, e.g. tert-butyl hydroperoxideand the sodium salt of hydroxymethanesulfinic acid or hydrogen peroxideand ascorbic acid, and very particular preference is given to combinedsystems which in addition contain a small amount of a metal compoundthat is soluble in the polymerization medium and whose metalliccomponent can exist in a plurality of valence states, for exampleascorbic acid/iron(II) sulfate/hydrogen peroxide, although in the caseof ascorbic acid it is also common to employ the sodium salt ofhydroxymethanesulfinic acid, sodium sulfite, sodium hydrogensulfite orsodium disulfite and instead of hydrogen peroxide it is common to employtert-butyl hydroperoxide or alkali metal peroxodisulfates and/orammonium peroxodisulfate. Instead of a water-soluble iron(II) salt it iscommon to employ a combination of water-soluble Fe/V salts. The amountof free radical initiator system used is preferably from 0.1 to 2% byweight, based on the total amount of monomers to be polymerized.

The manner of addition of the free radical initiator system to thepolymerization vessel in the course of the free radical aqueous emulsionpolymerization of the invention is rather of minor importance for thesuccess of the process of the invention. The initiator system can notonly be introduced into the polymerization vessel in its entirety aspart of the initial charge but also be added continuously or stepwise inthe course of the free radical aqueous emulsion polymerization at therate of its consumption. The choice in a particular case depends in theusual fashion not only on the chemical nature of the initiator systembut also on the polymerization temperature.

The polymerization pressure and the polymerization temperature arelikewise of rather minor importance. In general, the temperatureemployed will be between room temperature and 100° C., preferably withinthe range from 50° to 95° C. The employment of superatmospheric orreduced pressure is possible, so that the polymerization temperature mayalso exceed 100 ° C. and may in fact be as high as 130° C. Volatilemonomers such as ethylene, butadiene or vinyl chloride are preferablypolymerized under superatmospheric pressure.

The monomers mentioned by way of example as suitable for the freeradical aqueous emulsion polymerization of the invention are suitablenot only for use as constituents of the monomer mixture to bepolymerized, but also, in the same way as the free radicalpolymerization initiators and molecular weight regulators recommendedfor the free radical aqueous emulsion polymerization of the invention,for use as constituents of the starting polymer, although the monomer,regulator and initiator composition for the aqueous dispersion of thestarting polymer can be not only congruent with but also different fromthe monomer mixture to be polymerized according to the invention. Thisapplies mutatis mutandis necessarily also to the surface activesubstances to be used for preparing the aqueous dispersion of thestarting polymer, since in the process of the invention at least 50,preferably at least 60, % by weight of the total amount of surfaceactive substances contained in the final polymer dispersion of theinvention must already be part of the aqueous dispersion of the startingpolymer to be added. This amount of surface active substances need notof course have already been used in its entirety in the free radicalaqueous emulsion polymerization for preparing the aqueous dispersion ofthe starting polymer. On the contrary, some of it may also be addedsubsequently to the already fully polymerized aqueous dispersion of thestarting polymer.

Aqueous dispersions to be added according to the invention of a startingpolymer in which the starting polymer is present in the form ofdispersed starting polymer particles having a non-zero weight averageparticle diameter of less than or equal to 50 nm, preferably from 10 to40, particularly preferably from 25 to 35, nm are obtainable in aconventional manner. Appropriate teaching may be found for example inHouben-Weyl, Methoden der organischen Chemie, Volume E 20, part I,Makromolekulare Stoffe, Georg Thieme Verlag, Stuttgart, 1987, pages 248to 268. If the solids content is to be a usual 20-40% by weight, thestarting dispersions are obtainable in a particularly simple manner, forexample by mixing together the aqueous phase (which may contain forexample ammonia or NaHCO₃ for pH control), the monomers, the freeradical initiators (normally from 0.1 to 5% by weight, based on theamount of starting monomers to be polymerized), and surface activesubstances (customarily from 10 to 50% by weight, based on the startingmonomers to be polymerized) at a low temperature and heating the mixtureto the polymerization temperature and polymerizing it (the particle sizegenerally decreases with increasing surface active substance). Asregards the polymerization temperature and pressure the earlierstatements about the process of the invention apply. Of course, some ofthe aqueous dispersion of a starting polymer to be added according tothe invention can be prepared immediately beforehand in the samepolymerization vessel in which the actual process of the invention is tobe carried out. In fact there can be a smooth transition between the twoemulsion polymerization processes, in particular when this initialcharge portion of the aqueous dispersion of the starting polymer isprepared employing the stream addition method, in which part of thepolymerization batch is introduced as initial charge and heated to thepolymerization temperature and the remainder is then added to theongoing polymerization in a stream or spatially separate streams.

The number of dispersed starting polymer particles present in theaqueous dispersion of the starting polymer is obtainable in a simplemanner to good approximation by assuming a spherical shape for theparticles, determining the number average particle radius in aconventional manner (for example by photon correlation spectroscopy bymeans of dynamic light scattering or an analytical ultracentrifuge; thelatter is also suitable in particular for determining the weight averageparticle diameter and was employed here; cf. W. Machtle, MakromolekulareChemie 185 (1984), 1025-1039), then multiplying the polymer content ofthe aqueous dispersion of the starting polymer (determined by drying andweighing, minus the amount of emulsifier used) by three, and dividingthe resulting product by the product of four times the density of thestarting polymer, the ratio of the circumference of the circle to thediameter of the circle, and the third power of the number averageparticle radius. The process of the invention is preferable when thetotal number of starting polymer particles added dispersed in the formof at least one aqueous dispersion of a starting polymer is from 10¹⁷ to10.sup. 19 starting polymer particles per liter based on the volume ofthe final polymer dispersion. It is also preferable for aqueousdispersions of starting polymers to have a broad particle sizedistribution. Of course, in the course of the aqueous free radicalemulsion polymerization of the invention it is possible for there to bea change in the composition not only of the monomers to be added to thepolymerization vessel after the polymerization has started but also ofthe aqueous dispersion of the starting polymer to be added to thepolymerization vessel after the polymerization has started. Furthermore,the addition can take place not only stepwise but also continuously orby the gradient method. In general, a continuous mode of addition ispreferred.

In this way it is possible to obtain special structures for theparticles of the final polymer, as described for example in Kast,Makromol. Chem. Suppl. 10/11 (1985), 447-461. Preferred classes of finalpolymers are those polymerized

to an extent of from 70 to 100% by weight from esters of acrylic and/ormethacrylic-acid with alkanols of from 1 to 12 carbon atoms and/orstyrene,

or

to an extent of from 70 to 100% by weight from styrene and/or butadiene,

or

to an extent of from 70 to 100% by weight from vinyl chloride and/orvinylidene chloride,

of which the class of the acrylates is particularly preferred andpreferably includes the following monomer compositions:

95-99% by weight of at least one ester of acrylic and/or methacrylicacid with alkanols of from 1 to 8 carbon atoms, and

1-5% by weight of acrylic acid, methacrylic acid or a mixture thereof.

The process of the invention is preferably carried out by initiallycharging the polymerization vessel with that part of the polymerizationbatch which, apart from a possible polymerization initiator content,advantageously comprises only water and surface active substances and inaddition particularly advantageously with part of the total amount ofthe at lease one aqueous dispersion of a starting polymer to be added inthe course of the process, and it is a proposal of the invention thatthe total amount of surface active substance included in the initialcharge shall be at most 15% by weight, based on the total amount ofsurface active substances contained in the final polymer dispersion (ie.the aliquots A and B appearing in paragraphs i) to k) of the definitionof the final aqueous polymer dispersion of the invention given at thebeginning may have a common part). The initial charge is then heated tothe polymerization temperature and the remainder of the polymerizationbatch added to the ongoing polymerization. If the initial charge is freeof initiator and monomers the polymerization can be initiated forexample by starting the addition of monomers and initiator to the hotinitial charge at the polymerization temperature at the same time. Thefree radical polymerization initiators are advantageously added in aspatially separate stream, and their addition advantageously takes placeessentially synchronously with the addition of the monomers to bepolymerized. The monomers to be polymerized can be added not only bythemselves but also preemulsified in aqueous phase. If final aqueouspolymer dispersions having a particularly high solids volumeconcentration are to be obtained, it is preferable for the monomers nobe polymerized to be added by themselves.

It is advantageous when the addition of monomers to be polymerized andof the at least one aqueous dispersion of a starting polymer after thestart of the free radical aqueous emulsion polymerization of theinvention is effected in such a way that at any time of this additionthe polymerization conversion of the monomers already added previouslyto the polymerization vessel including the monomers introduced into thepolymerization vessel as initial charge is at least 90%, with particularadvantage at least 95%, based on the total amount formed of the monomersadded previously to the polymerization vessel and the monomersintroduced into the polymerization vessel as initial charge. It ispreferable when the addition of the at least one aqueous dispersion of astarting polymer ends not later than the end of the addition of themonomers to be polymerized, and it is particularly advantageous when,after the addition of the at least one aqueous dispersion of a startingpolymer has ended, up to 20% by weight of the total amount of the atLeast one monomer to be polymerized is further added to thepolymerization vessel.

If, after the start of the free radical aqueous emulsion polymerizationof the invention, the polymerization vessel is also charged with surfaceactive substances that are not part of the at least one aqueousdispersion of the starting polymer, then this addition is advantageouslyeffected in such a way that at any time the total amount of surfaceactive substances to be used, formed of the aliquots C and B alreadypresent in the polymerization vessel, is less than 3% by weight, basedon the total amount of monomers and polymer present in thepolymerization vessel. Preferably the addition takes place synchronouslywith the monomer addition. In the extreme case up to 50% by weight ofthe total amount of surface active substances contained in the finalaqueous polymer dispersion can be added after the actual polymerizationprocess has ended for the purpose of subsequent stabilization of thefinal aqueous polymer dispersion. Preferably, after the free radicalaqueous emulsion polymerization of the invention has started, thepolymerization vessel is not charged with any surface active substancesthat are not part of the at least one aqueous dispersion of the startingpolymer.

After the aqueous emulsion polymerization of the invention has startedthe addition of the at least one aqueous dispersion of a startingpolymer and of the monomers to be polymerized is advantageously effectedrelative to one another in such a way that the ratio V_(e) standardizedin paragraph g) of the definition of the final polymer dispersion of theinvention given at the beginning is at any time within the range >0 to 6and the ratio V_(a) standardized in the same paragraph g) is within therange from 0 to 7. Furthermore, the addition of starting polymerparticles and monomers after the free radical aqueous emulsionpolymerization of the invention has started is preferably effectedcontinuously.

On completion of the actual polymerization process the mixture ispreferably stirred for some additional hours while the polymerizationtemperature is maintained. This may be followed by customary measuresfor residual monomer removal, for setting a different pH or othermethods for post-stabilization.

Of course, the various possible, generally spatially separate, feedstreams can be mixed with one another immediately before entry into thepolymerization vessel.

The free radical aqueous emulsion polymerization of the inventiongenerally gives final aqueous polymer dispersions which have a very wideparticle size distribution which has all the following particle sizedistributions:

5-25% by weight of the final polymer≦200 nm

15-40% by weight of the final polymer≦400 nm

35-75% by weight of the final polymer≦600 nm

45-90% by weight of the final polymer≦800 nm

100% by weight of the final polymer≦1600 nm

This specific particle size distribution is presumably responsible forthe reduced flow resistance of the final aqueous polymer dispersions ofthe invention, which normally have Newtonian flow characteristics. Theparticle size distribution was determined in an analyticalultracentrifuge using the coupling PSD technique (see W. Machtle,Angewandte Makromolekulare Chemie 162 (1988), 35-42 (No. 2735)). Below asolids volume concentration of 50% by volume the effect of the particlesize distribution on the flow resistance decreases progressively.

The final aqueous polymer dispersions of the invention are generallyobtained as described in an industrially readily implementable mannerwith solids volume concentrations of up to 75% by volume with fullysatisfactory reproducibility and no flocs. The final aqueous polymerdispersions of the invention show their advantageous propertiesparticularly markedly at solids volume concentrations above 65% byvolume, which is why such final polymer dispersions are preferred. Theyare generally suitable for use as binders and as materials for preparingcoatings and adhesive joints, for which purpose they may haveadditionally mixed into them in a conventional manner assistants such asfilm forming aids, fillers or plasticizers.

EXAMPLES

1) Preparation of aqueous dispersions DS1 and DS2 of a starting polymer

DS1: a mixture of

65.4 kg of water,

25 kg of n-butyl acrylate and

22.25 kg of a 45% strength by weight aqueous solution of the surfaceactive substance corresponding to Dowfax 2A1

was admixed at room temperature first with a solution of 0.0625 kg ofascorbic acid and 0.005 kg of iron(II) sulfate in 2 kg of water and thenin the course of 3 minutes with a mixture of 2 kg of water and 0.65 kgof a 30% strength by weight aqueous hydrogen peroxide solution. Thetemperature was then maintained at 52° C. for 1 h. Thereafter themixture was cooled down to room temperature and following the additionof 0.05 kg of ascorbic acid in 1 kg of water additionally stirred for 2h.

The resulting aqueous dispersion of a starting polymer had a solidscontent of 30.2% by weight, a weight average particle diameter d_(w) of32 nm and a particle concentration of 1.7×10¹⁹ particles/kg.

S2: a mixture of

1800 g of water,

445 g of a 45% strength by weight aqueous solution of the surface activesubstance corresponding to the Dowfax 2A1,

10 g of a 25% strength by weight aqueous ammonia solution,

495 g of n-butyl acrylate, and

5 g of acrylic acid,

was heated to 80° C. and admixed with 25 g of sodium peroxodisulfateadded in one portion. The temperature was then maintained at 80° C. for1 h and the mixture was then cooled down to 25° C.

The resulting aqueous dispersion of a starting polymer had a solidscontent of 26% by weight, a weight average particle diameter d_(w) of 35nm and a particle concentration of 1.1×10¹⁹ particles/kg.

2) Preparation of various final aqueous polymer dispersions DF1 to DF4according to the invention

DF1: a mixture of

150 g of water,

22 g of a 20% strength by weight aqueous solution of the surface activesubstance corresponding to Dowfax 2A1,

and 11 g of aqueous dispersion DS1 from 1)

in a polymerization vessel was heated with stirring to 85° C. and thencontinuously admixed over 5 h at 85° C. with stream II. The start forthe addition of stream II was also the start for the continuous additionof stream I, the time-course of which was as follows:

4% by weight within 40 min,

then 8% by weight within 40 min,

then 88% by weight within 200 min.

80 minutes after the start of the addition of stream II an additional549 g of aqueous dispersion DS1 from 1) was started to be addedcontinuously according to the following schedule:

33.4% by weight within 80 min,

then 66.6% by weight within 120 min.

Stream I:

1345.2 g of n-butyl acrylate

889.2 g of methyl methacrylate

46.5 g of methacrylic acid

Stream II:

160 g of water

10 g of sodium peroxodisulfate

On completion of the addition of stream II the reaction mixture was heldat 85° C. for a further hour.

The product obtained was an essentially floc-free final aqueous polymerdispersion (2.93 liters) having a solids volume concentration of 67.6%by volume (75.4% by weight), a dynamic viscosity of 460 mPa.s determinedas per DIN 53019 at 23° C. and a shear gradient of 487 s⁻¹, and thefollowing particle size distribution:

7% by weight≦200 nm

18% by weight≦400 nm

55% by weight≦600 nm

85% by weight≦800 nm

95% by weight≦1000 nm

100% by weight≦1200 nm. DF2: a mixture of

160 g of water,

11.2 g of a 20% strength by weight aqueous solution of the surfaceactive substance corresponding to Dowfax 2A1,

and 11 g of aqueous dispersion DS1 from 1)

in a polymerization vessel was heated with stirring to 85° C. and thencontinuously admixed over 5 h at 85° C. with stream II. The start forthe addition of stream II was also the start for the continuous additionof stream I, the time-course of which was as follows:

4% by weight within 40 min,

then 8% by weight within 40 min,

then 88% by weight within 200 min.

80 minutes after the start of the addition of stream II an additional549 g of aqueous dispersion DS1 from 1) was started to be addedcontinuously according to the following schedule:

26% by weight within 80 min,

then 54% by weight within 60 min.

then 20% by weight within 60 min.

Stream I:

1915.2 g of n-butyl acrylate

114 g of ethyl acrylate

114 g of 2-ethylhexyl acrylate

114 g of methyl methacrylate

22.8 g of acrylic acid

3.4 g of tert-dodecyl mercaptan

Stream II:

160 g of water

10 g of sodium peroxodisulfate

On completion of the addition of stream II the reaction mixture was heldat 85° C. for a further hour.

The product obtained was an essentially floc-free final aqueous polymerdispersion (3.04 liters) having a solids volume concentration of 69.4%by volume (75.3% by weight), a dynamic viscosity of 375 mPa.s determinedas per DIN 53019 at 23° C. and a shear gradient of 487 s⁻¹, and thefollowing particle size distribution:

20% by weight≦200 nm

40% by weight≦400 nm

72% by weight≦600 nm

90% by weight≦800 nm

100% by weight≦1000 nm.

DF3: A polymerization vessel was charged with 175 g of water which werethen heated to 85° C. Then, starting at the same time, the followingstreams were added at 85° C. according to the following schedules:

Stream I:

2257.2 g of n-butyl acrylate

22.8 g of acrylic acid

2% by weight in 20 min,

then 4% by weight in 20 min,

then 8% by weight in 20 min,

then 86% by weight in 180 min.

Stream II:

667 g of aqueous dispersion DS2 from 1)

5% by weight in 60 min,

then 10% by weight in 60 min,

then 85% by weight in 120 min.

Stream III:

79.7 g of water

6.0 g of sodium peroxodisulfate

20% by weight in 5 min,

then 80% by weight in 115 min.

On completion of the addition of streams I and II the reaction mixturewas held at 85° C. for a further hour and then cooled down.

The result obtained was an essentially floc-free final aqueous polymerdispersion (2.9 liters) having a solids volume concentration of 69.5% byvolume (75.3% by weight), a dynamic viscosity of 430 mPa.s determined asper DIN 53019 at 23° C. and a shear gradient of 487 s⁻¹, and thefollowing particle size distribution:

16% by weight≦200 nm

30% by weight≦400 nm

35% by weight≦600 nm

45% by weight≦800 nm

60% by weight≦1000 nm

75% by weight≦1200 nm

90% by weight≦1400 nm

100% by weight≦1600 nm

DF4: As for DF3, but the stream addition schedules were as follows:

Stream I:

1% by weight in 20 min,

then 2% by weight in 20 min,

then 4% by weight in 20 min,

then 93% by weight in 300 min.

Stream II:

1% by weight in 5 min,

then 99% by weight in 295 min.

Stream III:

5% by weight in 60 min,

then 5% by weight in 60 min,

then 90% by weight in 240 min.

On completion of the addition of streams I and III the reaction mixturewas held at 85° C. for a further hour and then cooled down.

The result obtained was an essentially floc-free final aqueous polymerdispersion (2.92 liters) having a solids volume concentration of 70.3%by volume (75.9% by weight), a dynamic viscosity of 510 mPa.s determinedas per DIN 53019 at 23° C. and a shear gradient of 487 s⁻¹, and thefollowing particle size distribution:

23% by weight≦200 nm

35% by weight≦400 nm

65% by weight≦600 nm

90% by weight≦800 nm

95% by weight≦1000 nm

100% by weight≦1200 nm

We claim:
 1. An aqueous polymer dispersion whose solids volume concentration is ≦50% by volume and which has all the following polymer particle size distributions:from 5-25% by weight of the polymer≦200 nm from 15-40% by weight of the polymer≦400 nm from 35-75% by weight of the polymer≦600 nm from 45-90% by weight of the polymer≦800 nm 100% by weight of the polymer≦1600 nm,the polymer being formed from free radically polymerizable monomers.
 2. An aqueous polymer dispersion as claimed in claim 1, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 3. An aqueous polymer dispersion as claimed in claim 1, wherein said aqueous polymer dispersion comprises a polymer comprising:from 95-99% by weight of a C₁₋₈ alkanol ester of acrylic acid, a C₁₋₈ alkanol ester of methacrylic acid or a mixture thereof, and from 1-5% by weight of acrylic acid, methacrylic acid or a mixture thereof.
 4. An aqueous polymer dispersion as claimed in claim 3, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 5. An aqueous polymer dispersion as claimed in claim 1, further comprising at least one surface active substance of the formula I ##STR2## where R¹ and R² are each hydrogen or C₄₋₂₄ -alkyl, but are not both hydrogen, and X and Y are each an alkali metal or ammonium ion.
 6. An aqueous polymer dispersion as claimed in claim 5, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 7. An aqueous polymer dispersion as claimed in claim 1, wherein said aqueous polymer dispersion comprises a polymer comprising from 70 to 100% by weight of a C₁₋₁₂ alkanol ester of acrylic acid, a C₁₋₁₂ alkanol ester of methacrylic acid, styrene or a mixture thereof.
 8. An aqueous polymer dispersion as claimed in claim 1, wherein said aqueous polymer dispersion comprises a polymer comprising from 70 to 100% by weight of styrene, butadiene or a mixture of butadiene and styrene.
 9. An aqueous polymer dispersion as claimed in claim 1, wherein said aqueous polymer dispersion comprises a polymer comprising from 70 to 100% by weight of vinyl chloride, vinylidene chloride or a mixture thereof.
 10. An aqueous polymer dispersion as claimed in claim 7, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 11. An aqueous polymer dispersion as claimed in claim 8, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 12. An aqueous polymer dispersion as claimed in claim 9, wherein said aqueous polymer dispersion has a solids volume concentration of ≧65% by volume.
 13. An aqueous polymer dispersion whose solids concentration is ≧75.3% by weight and ≦75% by volume and which has all the following polymer particle size distributions:from 5-25% by weight of the polymer≦200 nm from 15-40% by weight of the polymer≦400 nm from 35-75% by weight of the polymer≦600 nm from 45-90% by weight of the polymer≦800 nm 100% by weight of the polymer≦1600 nm,the polymer being formed from free radically polymerizable monomers.
 14. An aqueous polymer dispersion whose solids volume concentration is ≧65% by volume and which has all the following polymer particle size distributions:from 5-25% by weight of the polymer≦200 nm from 15-40% by weight of the polymer≦400 nm from 35-75% by weight of the polymer≦600 nm from 45-90% by weight of the polymer≦800 nm 100% by weight of the polymer≦1600 nm,the polymer being formed from free radically polymerizable monomers selected from the group consisting of acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, acrylonitrile, vinyl halides, vinylidene halides, vinyl acetate, vinyl propionate, acrylamide, methacrylamide and vinylsulfonic acid and water soluble salts thereof. 